This is not a question; I'm posting this information because I've looked for it for ages and it's not self-explanatory.

In Cinnamon, on the Windows settings page, Behaviour tab, there is an option called "Attach dialog windows to the parent window". If this is enabled, dialog boxes will not only be in a fixed position, the parent window will also be covered in a dark overlay or grey / gray shadow. This means that if you're trying to adjust colours on an image in Pinta, you'll have to do it blindly because the image is too dark to see properly and you can't move the dialog box.
Disable it and it all works as it should.
It's not at all clear from the description of the setting that it'll do this.

Now that I've typed it down, I will probably never forget again. And maybe this will help someone else too!

I can't see what I'm doing!

So. Much. Better.

If your issue is solved, kindly indicate that by editing the first post in the topic, and adding [SOLVED] to the title. Thanks!

And that's a good default. Unfortunately it's easy to turn it on and not realise that the dark overlay will be a side effect, and then spend ages to find the problem. Now at least a forum search might turn it up!

If your issue is solved, kindly indicate that by editing the first post in the topic, and adding [SOLVED] to the title. Thanks!